How to Improve iPhone WiFi
Improving your iPhone's Wi-Fi will allow your phone to quickly stream video, download photos and surf the web. Wi-Fi connections provide your iPhone with a way to access a reliable connection that is frequently much faster than your cell phone carrier's 2G and 3G network speeds. When Wi-Fi connections are inexplicably slow, won't allow you to successfully connect to networks or you can't locate a Wi-Fi signal, you can take steps to improve your iPhone's Wi-Fi.
Restart your iPhone by pressing the "Home" and "Sleep" buttons simultaneously until you see the Apple logo appear on your phone's screen. The Apple logo indicates that your phone has restarted. Restarting your iPhone will not erase your files or change your preferences, but it will refresh your iPhone's memory and allow you to gain additional resources.
Charge your iPhone. Lower power levels can decrease your phone's ability to maintain a strong Wi-Fi signal, reducing overall signal strength.
Open your SIM tray by using the end of a paper clip in the tiny hole located on the SIM tray. Pull the SIM card out and re-seat the SIM card in the tray. Push the tray back into your phone. This will properly seat your SIM card in the SIM card tray and it will allow your phone to properly boot up and make a connection to your cellular carrier.
Reset your network settings. Tap "Settings" > "General" > "Reset" > "Reset Network Settings." After your iPhone restarts, all of your old Wi-Fi settings and DNS information will be deleted. This will allow your iPhone to get rid of bad DNS settings that might be causing connectivity problems. Your iPhone will also search for Wi-Fi networks in the area, and you will be required to re-enter all of your Wi-Fi settings again when you attempt to access a network.